An electrostatic powder spray gun with a high voltage, internal power supply, as illustrated and discussed in U.S. Pat. No. 5,056,720 ('720), assigned to Nordson Corp., the assignee of the present invention, which patent is incorporated by reference in its entirety herein, typically houses a power supply which can include an oscillator and a step-up transformer. Sections of the power supply can be housed either in the body of the gun or externally thereof depending on the specific operating requirements. An important benefit of housing the oscillator within the gun body is that each gun can be factory calibrated to match an external control unit. In the past, when the oscillator was not housed in the gun, each gun required field calibration. However, the oscillator generates heat and when the oscillator is housed within the gun body, means such as passive radiators with natural convection, of the type illustrated in the '720 patent, have effectively dissipated the heat generated in the gun, primarily by the oscillator. Until recently, the cooling requirements for the powder guns was not a significant problem because the powders commonly sprayed were not sensitive to the normal operating temperatures of the powder guns. Recently, however, new powder formulations have been introduced which tend to sinter together in a range of temperatures as low as about 95to about 100 degrees Fahrenheit (F.). Within this temperature range, even the relatively small amount of heat generated by a powder spray gun with an internally housed power supply is excessive. Thus, with the new powder formulations, heat transfer means such as passive radiators with natural convection are inadequate to cool electrostatic guns to a low enough temperature that prevents the sintering problem, particularly if the ambient temperature of the work area is in the range of about 85 to about 90 degrees F.
In powder coating systems, a jet pump or ejector is conventionally used to aspirate powder from a powder container or hopper and to transfer the powder through an outlet conduit to a spray device, i.e., a powder spray gun of the type disclosed in the '720 patent. The ability of a pump or ejector to control the flow rate of the powder is very important in order: a) to deliver the powder smoothly to the spray gun without surging or pulsing effects; b) to control the velocity at which the powder exits the spray gun; c) to insure that the air entrained powder is well dispersed in the air stream when it enters the charging or pattern forming structure in the spray gun; d) to minimize wear of the structural components of the gun; and e) to minimize impact fusion of the powder with the structural components of the spray gun. At present, powder pumping equipment attempts to accomplish these operating requirements with varying tradeoffs and varying degrees of success.
A conventional system for pumping air entrained powder from a container to a spray gun is illustrated in FIGS. 5 and 6, and primarily discussed beginning on column 6 line 47 to column 9, line 54, of U.S. Pat. No. 4,987,001 ('001), assigned to Nordson Corp., the assignee of the present invention, which patent is incorporated by reference in its entirety herein. A primary flow of air directed into pump 114 through an injector nozzle forms an air jet which creates a suction at a powder inlet. The suction at the powder inlet draws fluidized powder from a powder container 100 into pump 114 where it mixes with the air jet. The resulting air entrained powder is propelled through a venturi throat of an outlet pipe 116 to a spray gun. Varying the air flow through the injector nozzle 12 controls the suction and the volume of powder delivered to the spray gun. The air entrained powder can then be directed through an air amplifier 117 which injects a secondary flow of air to increase and precisely control the velocity of the air entrained powder flowing through outlet pipe 116.
While the secondary flow of air can be injected into the system at a location downstream of outlet pipe 116, as shown in the '001 patent, it is also known to inject the secondary flow of air at a location upstream from the powder inlet, see pages 32, 34-37 and 51 of PNEUMATIC HANDLING OF POWDERED MATERIALS, published by Constable & Company Limited, London, England, 1963, and incorporated in its entirety herein.
When the secondary flow of air is injected at an upstream location of the powder pump, the secondary air flow serves to "hold back" the powder and cause more air to be pumped. That is, since both the primary and secondary air flows pass through the venturi throat of outlet pipe 116, the throat velocity of the air entrained powder is higher then when only the primary air flows though the throat. Since impact fusion of powder and general wear of pump components vary in direct proportion to the square of the air velocity, the injection of the secondary air flow upstream from the venturi throat leads to both rapid wear of the pump components and impact fusion of the powder with the pump components.
In either case, i.e., when the secondary air flow is injected at an upstream or a downstream location from the powder pump, after the air entrained powder travels for a distance (usually about 4 to 12 meters) through outlet pipe 116 to the powder gun, the powder separates from the air stream for various reasons, such as the inertial separation effects of the bends in the conduit. To obtain a uniform spray pattern and achieve high electrostatic charging levels, the powder must be redispersed in the air stream before charging or pattern forming occurs. This redispersion can be accomplished by either adding additional air to promote strong turbulence and mixing, or mechanically inducing turbulence.
In some applications, such as when the powder stream is subdivided and distributed through multiple tubes of a triboelectric charging gun of the type described and illustrated in U.S. Pat. No. 4,399,945, which is hereby incorporated by reference in its entirety, the powder must be thoroughly redispersed in the air to insure that the powder is evenly distributed in the flow passage at the point of subdivision. Good results have been obtained with either air jet diffusers, as presently used in a Tribomatic II.RTM. gun manufactured by Nordson Corporation of Amherst, Ohio and described in U.S. application Ser. No. 07/956,615, filed Oct. 5, 1992, now U.S. Pat. No. 5,3344,082, which is also hereby incorporated by reference in its entirety, or with porous diffusers as shown in the '001 patent.
A number of serious shortcomings result when a powder spray gun with an air jet diffuser is operated in conjunction with a pump having a secondary air flow injected at either an upstream or downstream location, as previously discussed. First, there is an excess in both the volume and velocity of the air entrained powder being sprayed from the gun which lowers the overall coating efficiency and increases the overspray being generated and the amount of recycled powder introduced into the system. Second, the addition of a diffuser at the pump increases the control devices to three, i.e., one set of controls for each of the primary and secondary air flows at the pump and a third set of controls for the air flow through the diffuser mounted to the gun. Besides the extra cost associated with the additional set of controls, the adjustment of the three sets of controls to obtain the optimum settings is difficult and time consuming, especially for an inexperienced operator. Third, certain types of pattern forming elements and some powder charging schemes are not practical without very good powder dispersion at the gun.
Therefore, there is a need for a practical and easy to use system for pumping air entrained powder at a low flow rate to an electrostatic gun where the powder is redispersed in the air and the gun is cooled so that the new low temperature powder formulations can be effectively sprayed to apply a uniform coating on a workpiece.